How to disable the automatic UV stretching after an image size is changed?
Normally, it is not likely to happen. You need to learn more about how the UV coordinate space works. However, you can correct it by manually scale it down to the old proportion if you really want to keep the aspect ratio of that UV:

For example, let's say the old image resolution is 1000px * 1000px, and the new resolution is 500px * 1600px:


First, calculate two ratios: Rw={image_width_old}/{image_width_new} and Rh={image_height_old}/{image_height_new}. In this case, Rw=1000/500=2, and Rh=1000/1600=0.625.
Press A to select all UV elements, and SX2Enter, then SY0.625Enter. Done.


Furthermore, if you just simply unwrap it directly by some of the unwrap methods listed in the U menu (e.g. Project From View), then you can also consider do that again after switching to a different image, instead.  

I'm sure this can also be done by scripting to make it semi-automatic, at least. So, welcome anyone who know the scripting version of this method and edit my answer.